# Using ohmmeter to measure diode resistance

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#### moslema

##### Newbie level 3
Hi all,
Could the ohmmeter be used to read small resistance across a diode in the forward direction and large one in the reverse direction?
I tried that but I get strange results....the resistance was moderate and equal for both directions!!
Any explanation?

diode resistance

Diode is nonlinear device, and its resistance can not be measured by ohmmeter.
It has dynamic resistance (plus DC voltage) which is equal to
Req= dV/dI
If you want to know its effect on your circuit, it is better to model it by a voltage source (≈0.7V), if it is forward biased, and open circuit when it is reverse biased.

regards
amerion

Re: diode resistance

well when you connect a small resistance in parallel with a large resistance then it is going to have very little effect on the effective value of resistance...

so its effective value would depend on your choice of resistor... what is the value of resistor you chose and what is the output you got....

Re: diode resistance

u can instead try measuring the voltage and current through the diode using a multimeter at a particular value of bias and then divide them..

Re: diode resistance

A resistance meter will give you a resistance reading - but it will not be much use. This is what semiconductor curve tracers were invented for! If you know your meters bias voltage - or if you have a constant current one, you can get something out of the reading. If you just want to check the diode is working, then make sure the ohmeter has >0.7v to allow forward biassing and check for lower resistance one way, high the other - make sure you know which lead has the +ve.
I dont think this test works with ratio resistance measuring meters

Re: diode resistance

as it shows nonlinear resistance connect it to power supply . or in a ckt with minimum .7 v accross it then check for and rev res.

Re: diode resistance

Whenever you measure an out-of-circuit-diode with the multimeter in the diode-measure-mode, the value obtained is the actual voltage drop across the diode (Volts), not the resistance (Ω).

Re: diode resistance

It is also possible that the diode you are testing is faulty? What is the voltage rating of the ohmmeter you are using? Is the voltage at the probes sufficient to forward bias the diode?

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